In patients with human immunodeficiency virus (HIV) and other immunosuppressive diseases, one strategy for preventing infectious processes has been chemoprophylaxis or long-term suppression. However, drug toxicity and development of pathogen resistance have made this less attractive, as has the complexity of resulting drug regimens. Another approach would be detection of signal in serum or buffy coats by polymerase chain reaction (PCR) for pathogens such as MAC, cytomegalovirus (CMV), tuberculosis, phencyclidine, and perhaps others.PCR is being adapted for rapid screening. Initial efforts involve CMV. A quantitative assay involving buffy coats is being assessed in patients with HIV, organ transplants, and cancer. This protocol provides valuable resources for assay development. An assay that is fast, reproducible, and inexpensive is being tested.